1. Field of the Invention
This invention relates to steerable trailing arm suspensions for load-carrying vehicles. In one aspect, the invention relates to a steerable trailing arm suspension having a lift mechanism for raising an axle/wheel assembly to disengage the wheels from a ground surface. In a second aspect, the invention relates to a steerable trailing arm suspension with diverging trailing arms to retard lateral movement of the axle. In a third aspect, the invention relates to a steerable trailing arm suspension with a lockout device to prevent steering of the wheels.
2. Description of the Related Art
Several types of vehicles, including heavy-duty truck vehicles and the like, have multiple sets of axle/wheel suspension assemblies arranged in tandem so as to adequately support relatively heavy loads. To adjust the load support provided by these tandem suspension assemblies, it is known to employ suspension systems utilizing adjustably pressurized air springs and the like.
The suspension systems generally comprise identical suspensions mounted on opposing sides of the vehicle (preferably opposing parallel frame rails, extending along the longitudinal axis of the vehicle) and supporting the vehicle axle and wheel assemblies therebetween. The suspensions commonly comprise a hanger bracket and an air spring mounted to each frame rail and in a spaced relationship. One or more control arms, pivotally mounted to the hanger bracket, are pivotally mounted directly or indirectly to the air spring. Generally, an axle assembly is mounted to the control arms opposite the hanger bracket and the air spring mounts to the axle assembly. In this configuration, the axle assembly rotates with the control arms and the air spring dampens the motion of the axle assembly.
The axle assembly typically comprises an axle on whose opposite ends are rotatably mounted a knuckle yoke having a spindle on which a tire is mounted. The knuckles each have a tie rod arm, which are connected by a tie rod to link the movement of the opposed wheels and create a steerable axle. The steerable axle can be free steering or directly controlled.
In a steerable axle, the axle is often oriented at an angle relative to the ground to define a caster angle so that the wheels will follow the movement of the vehicle, reducing the scuffing and wearing of the tires and increasing the maneuverability of the trailer. The steerable wheels are very important during parking situations where high maneuverability is required. A disadvantage of the steerable wheel system is that when the tractor or trailer is operated in reverse, the caster angle is no longer appropriate for the direction of the travel and the wheels steer randomly and do not follow the direction of the trailer, making it difficult to back-up the trailer.
Previous solutions to controlling the steerable axle during the back-up generally fall into one of two categories. The first solution is to change the caster angle of the wheels, which is generally achieved by rotating the axle. The second solution is to lockout the wheels to prevent their rotation relative to the axle. One such lockout device is illustrated in U.S. Pat. No. 5,649,719 to Wallace et al., which discloses steering dampers connected between the steering arm and tie rod, and which have an internal valve which, upon closing, blocks fluid flow through the internal chambers of the steering damper to lock the relative position of the steering arm and the tie rods, preventing the turning of the wheels relative to the axle.
Another solution to the caster angle problem is to provide the suspension systems with a lift mechanism for lifting the suspensions from road engaging contact. When the vehicle is carrying a relatively light load, it is desirable to relieve the load transmitting relationship between the vehicle and one or more of the axle/wheel assemblies, and also to disengage the axle/wheel assembly from ground contact so as to reduce tire wear. But, the lift systems can also be used to lift the steerable axle when the vehicle is being driven in a reverse direction. To relieve load support when an air suspension system is employed, air pressure in the suspension can be reduced. To achieve disengagement of the tires from the ground surface, devices commonly referred to as axle lift mechanisms can be employed. Prior lift mechanisms utilize compressed mechanical springs located adjacent each wheel and mounted directly between the vehicle frame rails and the axle assembly. When the downward load forces exerted on the axle by the suspension system were relieved, such as through deflation of air springs, lifting forces exerted by the mechanical springs pulled the axle assembly upwardly to a raised position. An example of one such axle lift suspension is disclosed in U.S. Pat. No. 3,771,812 to Pierce et al., issued Nov. 13, 1973.
A disadvantage of the prior lift mechanisms is that the mechanical springs add a significant amount of weight to the lift axle suspension. Additionally, the constant upward force of the compressed mechanical springs add additional force to the air springs during operation., resulting in the need for a more robust air spring. The constant upward force of the compressed mechanical springs is counterproductive to the purpose of the air spring suspension, which is to keep the wheel in contact with the road surface.
Another disadvantage of previous lift axle suspensions is they have control arms that extend rearwardly from the hanger bracket and substantially parallel to the frame rails of the vehicle. Lateral forces imparted to the axle in these suspensions are resolved directly in the bushing, which pivotally connects the control arm to the hanger bracket. The bushing is designed to absorb rotational loads and the lateral loading results in unnecessarily increased wear and early deterioration of the bushings.